Unite said there was significant medical evidence that Mr Lyon was suffering from a serious stress-related illness as a result of the treatment he had "endured" at the hands of Grangemouth's owners Ineos.

The union said: "Unite believes that Ineos was determined to rush through a disciplinary process against Mr Lyon, denying his legal representatives the appropriate time to prepare his defence.

"Unite will be appealing the company's decision and issuing an employment tribunal claim for unfair dismissal and victimisation for trade union reasons."

Pat Rafferty, Unite's Scottish secretary, said: "Once again, a decent man and loyal employee has been hounded out of his job by Ineos.

"The rank hypocrisy of Ineos knows no bounds. The company withdrew its defamation action against Unite to 'draw a line under the dispute', yet at the same time it was drafting a letter dismissing a Unite convenor.

"Mark Lyon has been subjected to a grotesque mockery of the disciplinary system which saw him tried in his absence. While he attended a doctor's appointment, his employer was sacking him on trumped up charges. From start to finish, this has been a circus and reflects extremely poorly on Ineos.

"Ineos has run away from the defamation action and Unite stands by its assertion that Stevie Deans was victimised. Ineos has no interest in drawing a line and moving forward, just confrontation and conflict.

"We would urge Ineos to think again about a decision that is bound to send shock waves through the workforce. Worryingly skilled people are choosing to leave Grangemouth. Ineos needs to realise that this is not a time for further unrest and that they need to get the workforce on side to deliver a successful future."

Unite said the sacking would send "shockwaves" through the workforce, and warned that skilled workers are already quitting their jobs.